Hopefully I can be of some help, but your question might need to be narrowed down a bit for a better answer. I specialize in medieval medicine in Europe, so I can speak to that.

Although they did not have germ theory yet, they did understand that sickness traveled from one person to another, hence quarantine being the gold standard to prevent infection. This idea is traditionally called "miasma." When plague struck over and over again across Europe, town officials consulted with physicians and implemented quarantine as they saw fit. Plague doctor masks were stuffed with herbs that were thought to purify the air, which demonstrates this understanding of contagion. Doctors also used sticks to undress their patients to avoid touching them. Very basic, practical methods to avoid contagion.